Humans Have 46 Chromosomes; (23 Types), Where23 Come from the Father, 23 from the Mother

MEIOSIS Study Guide

This is a process that must occur with the gametes of sexuallyreproducingorganisms (that is, genetic material from a male and female of a species must come together to produce offspring), reducing chromosome numbers by half so two gametes can join together to make a “whole” number again:

Humans have 46 chromosomes; (23 types), where23 come from the father, 23 from the mother.

Regular body (somatic) cells have a 2Nnumber of chrom., 46, and divide by mitosis.

Gamete cells (egg/sperm) must have Nnumber of chrom., (½ the 2N number of chrom.)so when they join, the total number will be right (46). Meiosis makes this happen. It’s much like mitosis, but runs through the steps twice. So, the steps are called Prophase I, Metaphase I, Anaphase I, Telophase I, (NO Interphase here), straight into Prophase II, Metaphase II, Anaphase II, and Telophase II. Gamete cells arise from the meiosis of the germ cells, located in the gonads (testes/ovaries) of males and females.

Prophase I takes the most time, about 90% of meiosis. Interesting things happen here.
* The chromosomes have duplicated in S phase, just as before. Now, they contract and become visible.
* Homologous chromosomes pair up. These are the ones that are alike from your fatherand mother. For instance, chromosome #5 from your father and #5 from your mother come together. These would have the same genes on them. These 2 pairs of sister chromatids are called TETRADS. This process of
pairing up is called SYNAPSIS.

* Crossing over may occur here. This is when parts of non-sister chromatids break and reattach to the
chromatidbeside them. This is the first point in which DNA variation will occur in each gamete.

A meiosis I, B meiosis II,
C (interphase), D (prophase I), E( metaphase I), F (anaphase I), G (telophase I),
H (prophase II), I (metaphase II), J (anaphase II), K (telophase II),
L four haploid cells

Metaphase I
* Paired homologous chromosomes (called homologues) align in the center.
* Each one can align independent of any other. This allows Independent Assortment to occur. This is a random order in which the homologues position themselves to be separated. This is the second point in which DNA variation will occur in each gamete.

Anaphase I

* Homologue chromosomes (the tetrads) separate, move toward poles.
* Sister chromatids are still attached at their centromeres as doubled chromosomes.

Telophase I

* The newly forming cells now have a haploid number of chromosomes (although they’re still doubled as chromatids. Cytokinesis occurs.

Prophase II, Metaphase II, Anaphase II are just like MITOSIS.

Telophase II

* 4 haploid (N) daughter cells produced. This is sperm/egg or pollen/ovule (plants).

* In males, this process of making gametes through meiosis is called spermatogenesis.
Four sperm cells are produced at once through this process.

* In females, the process of making eggs is called oogenesis. Most of the cytoplasm goes to just one gamete. This will become the egg. The other three are very small, called
polar bodies. They will soon die.

Mitosis vs Meiosis